1. Field of the Invention
The present invention relates in general to the manufacture of color cathode ray tubes (CRTs), and more particularly to the adjustment of deflection yokes on CRTs, and most particularly to the calibration of test fixtures used to adjust deflection yokes on CRTs.
2. Background Information
A color CRT has red, green, and blue phosphors on the inside face. These phosphors produce red, green, and blue light when struck by an electron beam. Three electron guns are used to produce red, green, and blue images which are perceived as a full color image.
Each of the different colored phosphors is precisely placed on the face of the CRT and is separated from adjacent colors by a carbon strip that does not produce light when struck by an electron beam. The electron guns must be adjusted so that they accurately strike the appropriately colored phosphors to produce a high quality image.
In particular, the electron guns must be adjusted so that the full diameter of the electron beam falls on the phosphor rather than the carbon strip to produce maximum luminance. This adjustment is termed a landing calibration. Every dot on the screen is subject to a landing miscalibration and the landing calibration requires that the deflection yokes be adjusted to minimize the overall miscalibration.
Some CRT designs control each electron gun with a separate deflection yoke necessitating landing calibration of each yoke. Other CRT designs, such as the Sony Trinitron(copyright), use one deflection yoke to control all three electron guns and only a single landing calibration is required. Some CRT designs use phosphor dots and both a vertical and a horizontal landing adjustment are required. Other CRT designs, such as the Sony Trinitron(copyright), use vertical phosphor strips and only require a horizontal landing adjustment.
A landing adjustment jig can be used to adjust a deflection yoke. One such landing adjustment jig employs wobbling coils clamped around the neck of the CRT to deliberately shift the electron beams in response to a wobbling coil signal. The wobble causes the screen to get brighter and dimmer as the electron beam is swung through the point of being properly landed. Optical sensors check the luminance of the CRT under test at a plurality of points distributed over the face of the screen. The CRT is in calibration if the landing miscalibrations are minimized, as indicated by maximum luminance, when the wobbling coil signal is zero.
The landing adjustment jig is very sensitive and is able to detect landing offsets, mislandings, as small as 0.1 micron at each point on the CRT that is tested. The problem is that it is difficult to verify that the landing adjustment jig is functioning properly. What is needed is a way to verify the proper operation and accuracy of a landing adjustment jig.
The present invention is directed to a method and an apparatus that verify the correct operation and calibration of a wobbling coil CRT monitor landing adjustment jig. The invention receives a wobbling coil signal and a video test signal from the landing adjustment jig. The wobbling coil signal and the video test signal are disconnected from the landing adjustment jig while the invention is used. The invention modulates the received video test signal with the wobbling coil signal to produce a video test signal which is connected to the CRT under test to produce maximum luminance on the CRT under test at the time when the wobbling coil signal is such that a properly adjusted CRT under test would produce maximum luminance.